These investigations will be concerned with two processes of early development, the mechanism of chromosome movement during cell division and the breakdown of the cortical granules and formation of the hyaline layer at fertilization. The synchronously dividing eggs of the sea urchin will be used as experimental material and the aim is to elucidate the nature of these processes in macromolecular terms. Large quantities of isolated mitotic apparatuses (MA) can be isolated by methods already developed and their fine structure will be correlated with their protein composition so as to determine the structural role of the various components. The mobilization ofthe proteins of the microtubules and other components of the MA from the cytoplasm will be studied by comparing the structure and composition of MA isolated under various conditions to that in the intact cell. A major aim of this investigation is to obtain the isolated MA in a sufficiently native form that biological activity can be obtained in vitro. Other investigations will be concerned with the cortical granules of the sea urchin egg and their role in the formation of the hyaline layer at fertilization. The major protein of this layer has been isolated and purified and the mechanism of its packing into these granules and released at fertilization is now under investigation. Efforts will be made to isolate these granules in pure form so that these problems can be studied under extracellular conditions. The role of the hyaline layer, which acts to hold the blastomeres together during early development will also be studied by means of experimental modification of its rigidity and other physical properties.